Patent GB-785469-A (GB)
Title: Improvements relating to apparatus for testing electrical components
IPC: G01R27/28
Filed: 1955-01-28
Abstract: 785,469. Inductance and Q measurement. STEATIT-MAGNESIA AKT.-GES. Jan. 28, 1955 [Jan. 28, 1954], No. 2718/55. Class 37. Apparatus for measuring the Q or the inductance of an inductor (or the Q or the permeability of an inductor core), comprises a resonant circuit containing a test inductor (or an inductor with a test core), excited by the output of a variable frequency generator, a frequency comparison circuit comparing the resonant frequency of the resonant circuit with the frequency of the generator output to produce a voltage representing the frequency difference, and first and second control devices responsive to the comparison circuit output to respectively increase or decrease the generator frequency and restore coincidence with the resonant frequency, the first control device being operative only when the output of the comparison circuit is above a critical value at which the frequencies are equal, and the second control device being operative only when this output is below the critical value; and voltage and frequency measuring circuits respectively responsive to the voltage signal across the resonant circuit and to the generator frequency, operative to indicate values of inductor Q and inductance (or core Q and permeability), together with similarly responsive relays operating sorting devices. A variable frequency constant voltage H.F. signal generator 1 (Fig. 2) comprising an oscillator 15 tuned by resonant circuit 14 and reactance valve 2 followed by amplifier valve 16, 17 is capacitance coupled to a resonant test circuit 4 comprising the inductance LX to be tested and a variable tuning capacitance CK; the voltage across the test circuit at resonance with the constant voltage input signal being a measure of the Q of the inductance (or of its core) if the circuit losses are low. The H.F. voltage across capacitance CK of the resonant test circuit is capacitance coupled to limiteramplifying valves 19, 20 of a discriminator 7 comprising a special 7-grid valve 21 wherein the 1st and 7th grids are earthed, the 2nd, 4th and 6th grids are connected to an H.T. supply, the output of amplifier valve 20 is applied to the 3rd grid and the energizing signal voltage at the input of the resonant test circuit is capacitance coupled to the 5th grid. The anode current of valve 21 develops a direct voltage varying in amplitude and sense according to the phase deviation between the signals supplied to the discriminator, which is amplified in double triode 22 and applied to the grids of switching valves 5, 6 in series between a positive voltage source and earth so that the junction point of the valves regulates the magnitude and polarity of the charge on capacitor 3 and the screen grid of valve 2, whereby the tuning of circuit 14 and the frequency of the signal generator is adjusted to equality with that of the test resonant circuit 4. Test resonant circuit may be series arranged. A valve voltmeter 10 calibrated to indicate Q directly may be connected to the junction of CK and LX of the test circuit, and a voltage from the cathode of the voltmeter valve, may be amplified and applied to alternatively operate thyratrons 26 or 27 and relays 28 or 29 whereby inductors or cores under continuous test may be sorted into one of two bins according as they are greater or less than a preset Q value. The corrected output signal frequency from generator 1 is applied to discriminating resonant circuits 30, 31 tuned to preset frequencies corresponding to critical values of inductance LX (or permeability values of its core). The signal frequency being automatically tuned to the resonant frequency of LX CK, the voltages across circuits 30, 31 are arranged to operate respective thyratrons and relays 32, 33 to sort the inductors or cores under test into bins according to their inductance or permeability values.